As technology progresses, wireless devices tend to integrate more features and become increasingly ubiquitous. For example, wireless devices may include features associated with personal digital assistants (PDAs), cellular telephones, wireless internet access devices, global positioning system (GPS) receivers, and the like. Therefore, multiple radio frequency (RF) signals spanning a wide frequency range are common in many areas and may interfere with one another. Such wireless devices may be used in a number of different operating environments including operating indoors or outdoors. As a result, wireless devices may need to operate over a wide temperature range. Further, wireless devices may support one or more wireless communications protocols, such as third (3G), fourth (4G), or later generation cellular telephone protocols, GPS protocols, wireless fidelity (Wi-Fi) protocols, Bluetooth®, and the like. Some of these protocols may have tight channel spacings that require narrow channel bandwidths. Additionally, full duplex systems may transmit and receive RF signals simultaneously. In such systems, a difference between a transmit frequency and a receive frequency, called a duplex gap, may be narrow. As a result, a receive bandpass filter in a full duplex system may require a relatively narrow passband and may require a steep roll-off response when moving toward the transmit frequency to isolate receive circuitry from transmit signals. Further, since the duplex gap may be narrow, the receive bandpass filter may need to have low insertion loss in the passband toward the transmit frequency. In addition, to preserve signal integrity, a receive bandpass filter may need a clean response to minimize return loss.
Surface acoustic wave (SAW) devices are electro-mechanical devices that utilize surface acoustic waves, which travel along the surface of a substrate having some elasticity. The substrate has a relationship between an electrical signal and mechanical movement within the substrate. An applied electrical signal may be used to cause the mechanical movement, which may include surface acoustic waves, or the mechanical movement may be used to provide an electrical signal. Therefore, the substrate may be used in a transducer for transduction between an electrical signal and mechanical movement. An inter-digital transducer (IDT) is a special type of transducer used to convert an electrical signal into surface acoustic waves or to convert surface acoustic waves into an electrical signal.
A SAW track may include two or more IDTs along an acoustic cavity, such that the IDTs are acoustically coupled to one another. A first IDT in the SAW track may receive an input signal and a second IDT in the SAW track may provide an output signal based on surface acoustic waves that are coupled between the IDTs. Multiple SAW tracks may be electrically coupled to one another to form a SAW device, such as a SAW bandpass filter, which may be used as a receive bandpass filter in a full duplex system. Manufacturing costs of a SAW device may be directly related to the size of the SAW device, which is related to the sizes and spacings of IDTs used in the SAW device. Such sizes and spacings may be related to frequencies of surface acoustic waves in the SAW device. Thus, there is a need for a SAW device having a small size, which may be used in a receive bandpass filter having minimal insertion loss, having minimal return loss, having a steep roll-off response toward a transmit frequency, and having a wide operating temperature range.